A RING of steel is being erected in London in preparation for Margaret Thatcher's funeral, the city's biggest security operation since the 2012 Olympics.

Thousands of metal barriers were being erected along the route for her military funeral cortege from The Strand to St Paul's Cathedral to hold back crowds of people who want to support her and those who want to protest.

Operation True Blue, made up of officials from the police, Buckingham Palace and 10 Downing Street, met for last minute discussions on the expected mass demonstrations expected to blight what is supposed to be a solemn tribute to the British ruler who died last Monday aged 87.

Margaret ThatcherSource:Getty Images

The AUD$14 million security operation, the biggest since last year's London Olympics, will involve hundreds of police officers standing along the entire route which will go into lockdown at least four hours before the procession on Wednesday. All leave has been cancelled and extra troops have been brought in for local constabularies outside London.

They will be supported by mounted police unit, the police air helicopters and tactical riot squads. The metal barriers are seen as only a deterant and not likely to stop determined protest groups which police say could be hijacked by one of four recognised anarchist and anti-austerity groups hell bent on using the event, to be shown live across the world, to cause trouble.

A total of 16 people are in police custody after hundreds of demonstrators gathered at a London landmark to protest against Thatcher's legacy and mark her death with a party.

Metropolitan Police officers made the arrests - mostly for drunk and disorderly conduct - in the early hours of Sunday as crowds revelled in Trafalgar Square, Whitehall and surrounding streets.

Those arrested are aged between 18 and 44 and remain at a central London police station.

Of those arrested, eight suspects were taken into custody for being drunk and disorderly and two were arrested for assaulting officers.

Union members from across the UK, who had fierce battles with Thatcher in the 1980s, rubbed shoulders with those demonstrating against today's welfare cuts.

Despite the depth of feeling and a large police presence, there was no serious trouble.

A police spokesman said the last of the protesters left Trafalgar Square about 2am and there were no reports of any damage to property.

Britain Thatcher ProtestSource:AP

Operation True Blue has also discussed the potential for more serious attack from terrorists and Irish Republican militants and has also drafted in hundreds of armed forces including an SAS contingent, as well as those troops officially taking a role in the 700-personnel military funeral procession, for added security.

They too will line the route from Westminister to St Paul's along the Strand, Fleet Street and Ludgate Hill.

A tactical group is also being based on the River Thames to quickly deploy to any outbreak in trouble and are being kept in small units in side streets all of which will be blocked.Former senior police officer Roger Gomm said searches of the route are expected today (mon) with manhole covers and letter boxes checked and sealed.

Businesses and shops along the route have also been asked to be "eyes and ears" for anything suspicious in the days leading up to the event.

Britain Thatcher ProtestSource:AP

A whole police unit has been monitoring social-media for hints of trouble and so far have noted groups involved in running battles with police in 2009 during austerity protests have again been rallying supporters for a day of action.

Mr Gomm said however stunts were more likely.

"I think there is more likely to be a publicity stunt than terrorism, but if you were the police officer in charge you would want to have everything covered,'' he said.

"There are also a large number of visiting dignitaries some of whom do present their own security problems."